A crushing plant typically consists of a preliminary crusher, intermediate crusher and one or more after-crushers and screen decks. Depending on the number of after-crushers, the plant is called either two, three of four phase crushing plant. In four phase crushing plants, the second after-crusher may be replaced by an after-crusher for shaping of material.
Source material is fed with a wheel loader, a digger or a transfer vehicle to a feeder which measures out material to the feeder of the preliminary crusher. The product of the first crushing phase is transferred on a conveyor either directly to the intermediate or after-crusher or to the screen. In the second, third and fourth phase, crushing and screening is continued to prepare the desired end product.
The most common feeder type is a vibrating feeder that is used on a pre-determined basic speed. Usually, jaw crushers are used as preliminary crushers, usually gyratory crushers as intermediate crushers, gyratory and/or cone crushers are used as after-crushers. Screens are for example single-shaft free vibrating or multi-shaft directional impact screens.
At present, automation systems for crushing processes of mineral material are device-specific and not plant-specific or they are not at all available for mobile applications. To facilitate controlling the process, crusher-specific surface guards are used that work in an on-off fashion stopping/starting the feeding device (conveyor or feeder).
The present so-called on/off solutions do not optimise the productivity of the crushing process but the productivity of the plant depends to a great extent of actions made by an operator. The operator controls the speed of the feeder according to his ocular and empirical assessment. The operator also has to adjust the running parameters of the plant for each product and feed, separately manually case-specifically before starting the crushing process.
As the actions made by the operator directly influence the quantity and quality of the achieved end product, the operator's experience has a great impact in pursuing the desired crushing outcome. Inexperience in controlling the process weakens the crushing outcome regarding product capacity, desired particle distribution and quality.
The operator's concentrating in controlling the process is primarily important because even just a small slackening results in uncontrollability. For instance, when the feeder capacity exceeds the capacity of preliminary, intermediate or after-crushers it results in crushers flooding. For instance, when the feeder capacity is below the capacity of preliminary, intermediate or after-crushers it results in so-called idling of the crushers.
The operator's task is to create an even feed to the feeder so that the plant as a whole works at an optimal level. From the operator's point of view the control of the overall situation is further complicated by that material delivered to the feeder by a digger or a loading shovel often has to be collected from long distance in which case the feeder in the mean time becomes empty and functioning of the process weakens. Thus the operator does not have an easy task to keep the filling degree of the feeder at an optimal level.